Predicting polymer failures

Researchers from the Adolphe Merkle Institute’s Mechanoresponsive Materials group have developed an innovative method for analyzing mechanical stress and strain in polymers. This method uses mechanochromic materials, which change color or fluorescence when subjected to mechanical forces, to visualize and map these stresses at the microscopic level.

Understanding how defects in polymer materials react under mechanical stress is crucial for preventing catastrophic failures. Defects often cause localized stress and strain that can lead to crack initiation and propagation. Traditional methods of studying these phenomena lack the resolution to capture the intricate details at the molecular level. Mechanochromic mechanophores, which change their optical properties (color or fluorescence) in response to mechanical deformation, provide a promising solution. These mechanophores allow for the visualization of stress and strain distributions within polymers in real time. However, existing mechanochromic materials, typically based on rupturing covalent bonds, require significant force to activate and often provide irreversible signals, limiting their usefulness in detecting early-stage mechanical processes.

The research from the Mechanoresponsive Materials group introduces a novel protocol that combines optical microscopy, tensile testing, and image processing to create detailed maps of local strain around defects in polymers. This method uses mechanophores with weaker bonds that respond to smaller forces compared to pre-existing mechanophores, making them suitable for detecting low-stress mechanical processes. The scientists used three different supramolecular mechanophores to investigate their thesis. These mechanophores showed reversible optical changes under testing, enabling repeated measurements.

The researchers incorporated the mechanophores into polyurethane films, and then introduced circular defects. The films were then put under tension while optical changes were monitored with a fluorescence microscope. The researchers calibrated the mechanochromic response against the applied strain, allowing them to convert fluorescence images into maps of local strain distributions. This approach was validated using the three different mechanochromic systems and extended to study more complex strain distributions in polymers containing inorganic microparticles.

The study found that local strain around defects can differ significantly from the applied external strain. The strain maps revealed intricate deformation patterns that would be missed by traditional bulk measurement techniques. For instance, regions near defects exhibited much higher local strains, which are critical for understanding the onset of material failure. The method proved to be sensitive and versatile, capable of detecting both macroscopic and microscopic strain variations.

This new strain-mapping approach provides a powerful tool for studying mechanical deformation in polymers. By enabling detailed visualization of strain distributions at the microscopic level, it enhances the ability to predict material failure and design more resilient polymeric materials. The protocol is generalizable and can be applied to a wide range of polymers and mechanophore types, paving the way for potential broader adoption in material science research.

Reference: Traeger, H.; Kiebala, D.; Calvino, C.; Sagara, Y.; Schrettl, S.; Weder, C.; Clough, J. M. Microscopic Strain Mapping in Polymers Equipped with Non-Covalent Mechanochromic Motifs. Mater. Horiz. 2023, 10 (9), 3467–3475.